went
So I wonder what would happen if you were to reach level 100 the very first time you enter the Skull Caverns...
So now I think I may know the answer to my own question (halfway). I used the Skull Key for the first time and dove into the Skull Caverns the other day to go Prismatic Shard hunting. I managed to make it past floor 50 (but not 100) and went after every Iridium node I could find. I found a floor with like 10 Iridium nodes. 2 of them yielded Prismatic Shards -- jackpot! And now to see if I can roll that "Preserving" Enchantment on my Advanced Iridium Fishing Rod (but that's a story for another thread)...
Anyway, on my next game day, I received Qi's first Skull Cavern Quest in the mail -- to get to Floor 25. I hit the "Accept" button and immediately heard the "Quest Complete" sound. I did not venture into the Skull Caverns that day. The next game day, Qi sent me 10,000g.